Conventionally, there are various kinds of mechanical devices in which the unidirectional clutch is used. For example, in a paper handling mechanism for a copy machine, in order to prevent problems for the removing a paper at a time of a paper jam, there is provided the unidirectional clutch between a paper conveyance roller and a drive motor.
As for the unidirectional clutch, there is a well-known unidirectional clutch (for example, see Japanese Patent Documents 1 and 2) comprising an outer member including an internal gear; an inner member received in the outer member coaxially and freely rotatably, and also including a concave portion on an outer circumferential portion thereof; and a planet gear received freely rotatably in the concave portion, and meshing the internal gear. An engaging portion is provided in a concave portion side in such a way that at a time of a rolling movement in one direction of the planet gear which rolls and moves by a relative rotation of the outer and inner members, teeth of one portion of the planet gear engage; however, at a time of a rolling movement in the other direction, the engagement is released, so that torque is transmitted in one direction of the relative rotation of the outer and inner members, and the torque is not transmitted in the other direction.